I am seeing very similar behaviour, however, the rar file that contains the .sub file remains in the directory after everything else has been unrared, and clean up has occurred.
Because the subtitles generally are sorted to download after the main movie/tv episode, the .idx is correctly unrared. The rar file which contains the .sub has been renamed for example as showtitle.s01e01.720p.1.rar stays in the folder.
This means I then have to manually unrar the .sub file. I generally just use the right-click windows context menu for WinRAR - extract here.
What really needs to happen is a recursive unrar, specifically for the abcxyz-subs.rar file.
In my particular setup, although far from ideal, temp is on a separate drive, but everything downloads, par2 checks and unrars onto the one drive.
I might be able to change the setup of where the file downloads too and where the unrar destination is, but my download machine is really pushed for drive space - everything ends up on an external USB drive, which slows things down a tad further.
Thanks